The production of elongated metal forms is usually accomplished by casting metal into ingots and then rolling or extruding the cast metal. Many problems are encountered in preparing elongated metal articles in this manner. When rolling cast ingots, the amount of metal in the ingot limits the length of the article that can be made by it. For example, if a strip of indefinite length is to be prepared, it must be prepared by welding strips rolled from adjacent ingots together. In preparing long strip from cast ingots, a great deal of waste is encountered. The ingot itself must be scalped in order to obtain a good surface on the final rolled strip. Ends and sides of the strip must also be trimmed. Experience has shown that as much as 20% of the ingot must be returned to scrap in the process of preparing strip from it.
In all metals, and particularly with alloys, the relatively slow cooling of massive ingots causes differential composition across the cross section of the metal. A slowly cooling alloy will experience fractional crystallization so that the composition of the quickly cooled shell is different from the composition of the more slowly cooled center of the ingot. Even if the composition is the same throughout the cross section of the ingot, the slowly cooled center will have a different grain structure than the shell as a result of different cooling rates. These composition and structural differences show up in end-to-end variation in hot rolled band; and, as mentioned hereinabove, the problems encountered are particularly serious in the preparation of alloys.
Powder metallurgy techniques avoid some of these problems but create others. Powder metallurgy techniques involve atomizing, cooling, screening, cold-compacting, and then heating and hot-compacting of the powder particles. These many processing techniques require high capital and equipment investment. Additionally, in some powder metal processes the surfaces of the individual powder particles are oxidized so that the compacted form contains large quantities of metal oxide. The compacted powder metal from these processes, although very strong, is still in the form of a sintered mass; and it does not have the density or the strength of the parent metal.
Other powder metal processes produce a product at the density of the parent metal, but these processes are very expensive, requiring inert atmospheres and the use and removal of cans. Furthermore, they cannot be used to make strip of indefinite length.